The acid phosphates and arsenates of the tetravalent metals, of general formula M(HAO.sub.4).sub.2.nH.sub.2 O (A=P, As; M=Zr, Ti, Hf, Ge, Sn, Ce, Th) have been obtained not only as amorphous materials but also as semicrystalline and crystalline materials, both with a fibrous structure and with an .alpha. and .gamma. stratified structure.
These materials, and in particular zirconium acid phosphate, have been and are still the subject of intense research in various national and foreign laboratories because of their stability towards acids and/or oxidising agents, their high resistance both to temperature and to ionising radiation, and their potential application in the field of ion exchange, heterogeneous catalysis, intercalation of molecules with proton acceptor groups, solid electrolytes and chromatograph supports.
A compendium of the enormous amount of experimental work carried out up to the present time can be found in certain magazines (see for example G. Alberti in Account of Chemical Research, 11, 163, 1978) and in some recent monographs in scientific tests [see for example the first three chapters of "Inorganic Ion Exchange Materials" by A. Clearfield, G. Alberti and U. Costantino respectively (Editor Clearfield) CRC Press USA 1982, and Chapter V by G. Alberti and U. Costantino in "Intercalation Chemistry" (Editors Whittingham and Jacobson) Academic Press, USA 1982].
Zirconium acid phosphate, .alpha.-Zr(HPO.sub.4).sub.2.H.sub.2 O, has a stratified structure with a distance of 7.56 .ANG. between layers. Each layer is constituted by a plane of zirconium atoms sandwiched between two layers of O.sub.3 P--OH tetrahedron groups. The structure is such that each zirconium atom is octahedrally coordinated with six oxygens of six different O.sub.3 P--OH groups. The distance between two O.sub.3 P--OH groups on the same face of any layer is 5.3 .ANG., so that the free area connected with each group can be evaluated at about 24 .ANG..sup.2. The various layers are packed together in such a manner that in the interlayer region cavitites of the zeolite type form interconnected by windows having a maximum diameter of 2.64 .ANG.. Ion exchange processes for ions which are larger than this size, or intercalation processes for molecules of large cross-section are either impossible or take place with extremely slow kinetics.
This drawback has been partly obviated by substituting 50% of the protons by sodium ions. This forms zirconium phosphate monosodium pentahydrate ZrHPO.sub.4.NaPO.sub.4.5H.sub.2 O, which has a distance of 11.6 .ANG. between layers. The sodium ion is then successively substituted by larger ions. It has been surprisingly found that the aforesaid drawbacks of the known art can be obviated by using an inorganic cation exchanger in which part of the phosphate (or arsenate) groups has been substituted by phosphite groups, and it has also been surprisingly found that the substitution leads to solid solutions containing both phosphate (or arsenate) ions and phosphite ions.